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Old 11-30-2010
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Another Yanmar Low RMP Issue

I have a 2008 Catalina 309 with 3 cyl. Yanmar. It's a Shared Use (WindPath) boat with 150 hours. Engine recently lost power. Would only run at 1900 rpm and not much more in neutral. Checked prop, no fowling, turned easily. I notice I had not change the Fuel Filter for awhile so change primary Raycor with no benefit. Then changed the secondary filter on engine. Engine rpm increased to 2400 but would not go to typical cruise rpm (3000+). In neutral, engine reved to 3200. Checked air filter-no change with filter removed. Checked for air leaks, tightened fittings, checked injectors (all working). After 3 weeks, max rpm is 2100, slight black smoke coming from exhaust when throttle pushed further. Runs smooth. Only about 2500 in neutral. I read posts below on filter clogged, prop question-not applicable. Other low rpm post-no help. Any ideas? Could the injectors be partially plugged due to the engine filter getting too dirty? Could the fuel be that dirty and plug filters again? I am planning to change filters again and see what happens. Planning to add a fuel treatment.
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Old 11-30-2010
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If the prop hasn't changed, and isn't fouled, and the propshaft isn't fouled, and the filters have all been checked or replaced...one other thing it can be is the fuel delivery system.

Have you checked the fuel pickup tube? On many boats, there is a screen covering the end of the fuel pickup tube, and these can often get fouled by "algae" or other debris in the fuel tank. Removing the screen and letting the fuel filters deal with the stuff is usually the best way to get around this problem.
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Old 12-01-2010
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My tank had the screen on the fuel outlet tube and it was clogged. To clear it I pulled off the hose and blew into it clearing the screen. I later pulled the tank and cleaned it. Had a lot of black gunk in it. The screen is not a good design on the Moeller tank (what I have). The tube is about 1/2 inch ID with the fine screen fitted over that. That gives about 0.8 squre inches of screen filter area- not much. I pulled the fill hose off my tank and looked in with a flash light. I could see the gunk on the draw tube screen plus all the gunk at the bottom of the tank. I plan to polish my fuel once a year myself by pumping and filtering then return to tank. I also may remove the screen and install an inline fuel strainer upstream of my racor just to catch the big gunk. I also plan to install a fuel vacuum gage at the engine- this will tell me the condition of the fuel system downstream of the yanmar mechanical pump (install gage just downstream of the yanmar fuel pump).
Good luck
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